A widespread consumption of information-intensive multimedia requires high-resolution, high-contrast, wide-color displays with a blur-free video. Traditional transflective LCDs (liquid crystal displays) provide legibility in a wide range of illuminances but are difficult and expensive to implement at high resolutions. Emissive displays such as organic light-emitting diode displays (OLEDs) and transmissive LCDs provide a good color and high resolution, respectively, but suffer from low contrast at high illuminances. Outdoor contrast can be improved by increasing display luminance but at the expense of the higher power consumption and/or permanently reduced color saturation. For display contents such as text-viewing and web-browsing, however, color saturation requirements are relaxed, and hence the outdoor luminance contrast can be increased by deliberately sacrificing the color reproduction (the reflective mode of transflective displays also has low color saturation). When indoors, such an approach will instead result in lower power consumption because a sufficent luminance contrast is possible to achieve with lower backlight power.
For other applications, the color reproduction may be more important such that the power consumption can be sacrificed for higher color saturation. Blur-free video can only be achieved with an intermittent backlight, which, however, inevitably reduces the average luminance and hence contrast in the outdoors. Also in this case, the luminance can be gained by deliberately reducing the color gamut.
Conventional LCDs and OLEDs are spatially divided into picture elements (pixels) which, in turn, are spatially divided into individually addressable subpixels which represent each primary color, e.g., RGB (red, green, blue). In the case of LCDs, white light from the surroundings (reflective displays) or from the backlight (transmissive displays) is filtered through primary colour filters on the subpixels to form pixels of any color. Field sequential color displays (FSCDs) are transmissive displays without subpixels or color filters and the image is instead formed by a sequence of images separated into each primary color, e.g. RGB. This sequence is faster than the integration time of the human visual system (HVS) so the colors are “fused” in the brain.
Transmissive LCDs are desirable for high-resolution displays above 300 pixels-per-inch, e.g., 2.4″ 480×640 displays. However, pixel aperture ratio decreases by increased resolution, resulting in increased optical losses. Moreover, dense color filters are required for saturated colors but their large absorption together with small aperture ratio results in low luminance and hence low contrast in the outdoors. The FSCDs has a larger aperture ratio because the pixel area is not divided into three primary color areas. Neither does it use absorbing color filters but each color is, on the other hand, only displayed during maximum 1/Nth of the time where N is the number of primary colors. In addition, primary color LEDs, e.g. RGB, have a lower luminous efficiency compared to white LEDs used in color filter-based displays.
Compared to conventional transmissive LCDs, the FSCDs feature blur-free video thanks to the intermittent nature of the backlight. Also, their resolution is N times higher than a color filter display and the number of primaries is scalable, even after the display has been fabricated. However, the LEDs of FSCDs have lower luminous efficiency so higher power consumption is required to achieve adequate outdoor contrast.
High moving image quality is generally achieved by reducing the frame duty, e.g., reducing the fraction of the frame or field during which an image is displayed, but at the expense of the average luminance. The sequential displaying of each primary in FSCDs also inevitably leads to color break-up, e.g., brief colored flashes when the terminal is shaken or when the gaze point is changed across the display. Color breakup also manifests itself as colored edges of moving objects when tracked by the eyes.
White point adjustment of a display is usually done by bending the gamma curves but this results in a bit depth loss via gray shade compression, i.e., only a part of the addressable colors are distinguishable.
Primary-color LEDs used in FSCDs exhibit a larger manufacturing spread in luminance and wavelength than white LEDs and hence a larger spread in display white point. Finally, FSCDs or any display with intermittent backlight is a subject to a flicker at sufficiently low frame rates and/or high luminances.
The luminance problem of FSCDs has been attempted to be resolved by adding more LEDs, overdriving the existing ones or selecting LEDs with less color saturation. However, this typically results in higher cost, shorter LED life time, higher power consumption, as well as permanently lower color saturation, respectively.
For example, one way to increase luminous efficiency of LCDs is to employ an extra white primary “color”. This has been proposed both in the spatial domain (RGWB subpixels) by B.-W. Lee, K. Song, Y. Yang, C. Park, J. Oh, C. Chai, J. Choi, N. Roh, M. Hong, K. Chung, S. Lee, C. Kim, “Implementation of RGBW Color System in TFT-LCD”, Paper 9.2, p 111, SID Digest (2004), and in the temporal domain (RGBW color fields) by Y. Toshiakaki, B. Keiichi, M. Tesuya and T. Shinji, Japanese patent application JP-2002-318564. While this provides 50% higher luminance for full white, it also results in 25% lower luminance of fully saturated pixels, assuming the same backlight. Another approach (e.g., see Pentile Matrix technology by Clairvoyante Laboratories, www.clairvoyante.com) is to spatially sub-sample blue utilizing the lower retinal resolution in the blue and lower contribution to the luminance. All these approaches suffer from a fixed spatial/temporal pattern and is therefore less flexible when trading off luminance for gamut.